Oil-injected vs Oil-free Rotary Screw Air Compressors: What’s the Difference?

When selecting a rotary screw compressor, one of the most important decisions is choosing between an oil-injected (also called oil-lubricated) and an oil-free model. Both types serve the same fundamental purpose—supplying compressed air—but they differ significantly in design, operation, cost, and suitable applications.

This article explores the key differences between oil-injected and oil-free rotary screw air compressors to help you make an informed decision for your business.

What Is an Oil-injected Rotary Screw Compressor?

Oil-injected air compressors use a small amount of lubricating oil that is injected into the compression chamber. This oil serves three main purposes:

• Lubricates the rotors for smoother operation
• Seals the gaps between the rotors to increase efficiency
• Cools the air during compression to prevent overheating

After compression, the oil-air mixture passes through a separator where most of the oil is removed and recycled. The air then continues to an aftercooler before being delivered to the application.

What Is an Oil-free Rotary Screw Compressor?

Oil-free rotary screw compressors do not use oil inside the compression chamber. Instead, they rely on:

• Precision engineering with tight rotor tolerances
• Special coatings (e.g., PTFE or ceramic)
• Alternative materials like water or air for cooling (in some designs)

These compressors deliver clean, oil-free compressed air, which is essential for sensitive industries like food, pharmaceuticals, and electronics.

Key Differences Between Oil-injected and Oil-free Compressors

Which One Should You Choose?

Choose Oil-injected if:

• You need a reliable and cost-effective solution for general industrial use
• Your compressed air quality requirements allow minimal oil traces
• You prioritize lower initial investment and higher energy efficiency

Choose Oil-free if:

• You operate in a regulated industry where air purity is critical (ISO 8573-1 Class 0)
• You want to avoid downstream filtration and contamination risks
• You can justify the higher investment for clean air and long-term reliability

Industry Applications

Long-term Considerations

Total Cost of Ownership (TCO)

• Oil-injected compressors have a lower purchase price and high energy efficiency, but higher maintenance costs due to oil changes and separator replacement.
• Oil-free compressors cost more upfront but offer long-term savings in applications where oil contamination would lead to product loss or production downtime.

Environmental Impact

• Oil-free compressors are often chosen for environmentally conscious operations.
• They reduce the need for oil disposal and minimize the risk of oil leaks or contamination.

Conclusion

Choosing between an oil-injected and oil-free rotary screw compressor depends on your industry, air quality requirements, and budget. Oil-injected compressors are ideal for general industrial use, while oil-free models are essential for clean, contamination-free operations. Understanding the differences will help you invest in the right technology for maximum performance and peace of mind.

Reference

1.Oil-free vs. Oil-injected air compressors. https://www.atlascopco.com/en-sg/rental/resources/blog/oil-free-vs-oil-injected-air-compressors

2.The Difference Between Oil-Injected and Oil-Free Rotary Screw Compressors. https://www.airmac.com/difference-oil-injected-oil-free-rotary-screw-compressors/